Azimuthally sensitive resistivity logging tool

ABSTRACT

Various systems and methods for implementing an azimuthally sensitive resistivity logging tool are disclosed. One such method involves transmitting a primary magnetic field from one or more coils placed on a drill collar and receiving several electrical signals, where each of the electrical signals is received from a respective one of several sensors. The sensors are distributed around a circumference of a drill collar, and an axis of at least one of the sensors is perpendicular to an axis of the drill collar. Each of the electrical signals indicates a respective magnitude of a measurement of a reflected magnetic field, where the reflected magnetic field is reflected from an anomalous geological formation. The method calculates a vector measurement of the reflected magnetic field, based upon the electrical signals.

FIELD OF THE INVENTION

This invention relates to the field of logging while drilling, particularly as used in oil and gas development and exploration.

DESCRIPTION OF THE RELATED ART

Logging is a technique that is used to measure one or more characteristics, such as resistivity, of subsurface geologic formations. Such a measurement can be used, for example, to determine the type of subsurface formation surrounding a drill bit. Accordingly, logging provides useful information to engineers and geologists engaged in hydrocarbon exploration and production as well as similar fields, such as mining.

Logging can be performed by inducing a current to flow in a formation and then selectively measuring the current distribution. Several different techniques for performing logging have been developed. For example, open-hole logging involves in removing the drill pipe and bit from a wellbore and then lowering an open-hole logging tool into the wellbore to obtain the desired measurements.

Logging-while-drilling (LWD, also known as measurement-while-drilling (MWD)) systems have also been developed. These systems differ from open-hole logging in that measurements can be obtained while the drill pipe is in the wellbore. LWD systems permit log information, such as resistivity, to be measured in a formation very soon after the formation is penetrated by the drill bit. This provides substantially “real-time” information that (a) is obtained before the formation is substantially altered by inflow of drilling fluids or other factors and (b) may be used by the driller to control the drilling operation, for example by steering the bit so as to penetrate (or so as not to penetrate) a selected formation detected by the LWD system. LWD systems typically include transmitters and sensors disposed in or on sections of drill pipe that are located near the drill bit.

Some existing LWD systems have developed techniques to determine whether the drill bit is approaching an anomaly within a formation. However, these techniques generally lack the ability to pinpoint the location of the anomaly relative to the drill bit. Such techniques also lack the ability to distinguish between a more conductive anomaly on one side of the drill bit and a less conductive anomaly on the other side of the drill bit. In addition, existing techniques typically employ sensors that are responsive in only one rotational direction, requiring rotation of the entire drillstring to detect an anomaly, which may be inconvenient and time-consuming. Accordingly, improved techniques for use in LWD systems are desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be acquired by referring to the following description and the accompanying drawings, in which like reference numbers indicate like features.

FIG. 1 illustrates a drilling system in which embodiments of the present invention can be employed.

FIGS. 2A-2C illustrate example sensor arrangements on a logging-while-drilling (LWD) tool, according to various embodiments of the present invention.

FIG. 3 is a cross-sectional view of the LWD tool, illustrating an example sensor arrangement, according to one embodiment of the present invention.

FIG. 4 is a flowchart of a method of operating the LWD tool, according to one embodiment of the present invention.

FIG. 5 is a block diagram of components that can be included within and/or coupled to the LWD tool, according to one embodiment of the present invention.

FIG. 6 illustrates the magnetic fields that can be induced and measured by one embodiment of a LWD tool.

FIG. 7 illustrates how a vector representation of a reflected magnetic field can be calculated, according to one embodiment of the present invention.

FIG. 8A illustrates a view of a LWD tool showing how the receiver sensor can be placed in the wall of the drill collar, according to one embodiment of the present invention.

FIG. 8B illustrates another view of the LWD tool of FIG. 8A.

While the invention is susceptible to various modifications and alternative forms, specific embodiments of the invention are provided as examples in the drawings and detailed description. It should be understood that the drawings and detailed description are not intended to limit the invention to the particular form disclosed. Instead, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

FIG. 1 illustrates how a drilling operation employs drilling rig 10 to cut a borehole 12 into the earth, penetrating the subsurface geological formation. Drillstring 16 passes through borehole 12 and is coupled between drilling rig 10 and a drill bit 14. Drillstring 16 includes drill bit 14, drill collars 28, and drill pipe.

The lowest part of drillstring 16 is made up of drill collars 28. Drill collars 28 are heavy walled pipe that provide weight on drill bit 14 and strength to resist buckling under their own weight. The drill pipe is thinner walled. The drill pipe is kept in tension (which may be effected by collars 28 placing weight on drill bit 14) to prevent buckling. Drill collars 28 may have radial projections (not shown) called stabilizers. Short drill collars, which may be adapted for specialized functions, are called “subs,” and references herein to drill collars are intended to include subs.

Drilling rig 10 turns drill bit 14, which cuts through the rock at the bottom of borehole 12. In some situations, drilling rig 10 turns drill bit 14 by attaching drill bit 14 to the lower end of drillstring 16 and turning drillstring 16 with powered equipment at the surface. Alternatively, as shown in FIG. 1, drill bit 14 may be driven by a motor 18, which is adjacent to drill bit 14 in borehole 12, through bent sub 20. The illustrated arrangement is known as a “steering tool” system, in which drillstring 16 does not need to rotate to turn the drill bit. However, drillstring 16 can be turned to steer drill bit 14, so as to control the direction of advance of borehole 12, thus permitting the route of borehole 12 to be precisely steered as desired through the subsurface geologic formation.

A logging-while-drilling (LWD) tool 22 is placed in drillstring 16, near drill bit 14 (if LWD tool 22 is used for geosteering, it may be desirable to locate LWD tool 22 as close as possible to drill bit 14). In a steered system, the LWD tool may be placed above mud motor 18, such that LWD tool 22 receives power and returns data to the surface through a wire line cable 24 that is passed down the center of a non-rotating (or slowly rotating) drillstring 16. In a system that uses a rotating drillstring 16 to turn drill bit 14, LWD tool 22 may be placed just above drill bit 14, and a mud pulse data telemetry system (or any other appropriate telemetry method) can be used to return information to the surface. Thus, LWD tool 22 is operatively positioned in borehole 12, typically with an annular space (e.g., filled with drilling mud) between tool 22 and the borehole wall.

LWD tool 22 can incorporate or be associated with directional sensors 26 that provide directional information to the driller to assist in controlling the steering of the drill bit. For example, such directional sensors can be calibrated to indicate the position of the LWD tool 22 relative to an absolute direction, such as the gravity vector or magnetic north.

LWD tool 22 also includes several receiving sensors, such as coils, that are each configured to measure a reflected magnetic field, as well as one or more transmitter coils that are configured to generate a magnetic field. In operation, LWD tool 22 generates a magnetic field, which is output by one or more transmitter coils. This magnetic field passes through the surrounding subsurface geologic formation and, if an anomalous formation is present, is reflected by that anomalous formation. The reflected magnetic field, if any, is detected by each of the receiving sensors included within LWD tool 22 (these sensors can, in at least some configurations, also detect the primary magnetic field induced by the transmitter coils). Since each sensor is located at a slightly different location, each sensor may detect a different magnitude of the magnetic field.

The portion of the magnetic field that is received by each sensor is sensed and quantified by electronics within LWD tool 22. The magnitude of the detected magnetic field has an inverse relationship to the formation's resistivity in proximity to the receiving sensor. Thus, the quantified detected magnetic field can be converted to information that identifies the resistivity (or conductivity, which is simply the inverse of resistivity) of the proximate portion of the formation. Additionally, differences between the quantified reflected magnetic field, as seen at each of the different sensors, can be used to determine the azimuthal direction, relative to the position of the drill collar, to an anomalous formation. LWD tool 22 can also include or be coupled to telemetry or other communication equipment to transmit this information to the earth's surface.

Above the earth's surface 30, telemetry receivers and/or other appropriate communication equipment can be located in a logging truck 32 located near drilling rig 10. Thus, communication equipment is positioned to receive and interpret the information generated by LWD tool 22 and directional sensors 26, so that the information can be collected for later analysis and/or used to steer wellbore 12 into the desired position (e.g., to maximize recovery of hydrocarbons from a selected reservoir).

A data display panel 34 can be provided on or near drilling rig 10 and/or logging truck to give an operator (e.g., a driller, engineer, geologist, or the like) real-time information about the directional progress of wellbore 12 as well as the formation properties of the geologic formation currently near LWD tool 22. In one embodiment, data display panel 34 can be part of a computing device (e.g., data display panel 34 can be rendered on the screen of a laptop computer used by an operator of drilling rig 10). Data display panel 34 can provide a polar display indicating formation properties of an anomaly within the geologic formation. For example, information identifying (or usable to identify) the resistivity (or another electrical characteristic) of such an anomaly can be provided by LWD tool 22. This information can be processed in order to render a polar display (or other graphical user interface) showing the orientation of and/or distance to the anomalous formation from the borehole (e.g., by showing the borehole in the center of a polar display and rendering a visual representation of the location of the anomalous formation relative to the borehole).

FIGS. 2A-2C illustrate example sensor arrangements for LWD tool 22. In each example, there are several transmitting coils, which can be used to induce a magnetic field in the surrounding formation, and several receiving sensors, which can each detect the induced magnetic field and/or a reflected magnetic field. Many other sensor configurations are possible in addition to those shown herein.

LWD tool 22 can be implemented as a sub (e.g., a drill collar) for use as part of a drillstring, as described above. In one embodiment, the structural features and physical components of LWD tool 22 are similar to those described in U.S. Pat. No. 6,064,210, which issued on May 16, 2000 and names Paul L. Sinclair as inventor, and which is hereby incorporated by reference as if completely and fully set forth herein.

The transmitter coils described herein are induction tools (e.g., inductors) that create an alternating magnetic field that propagates around and/or away from LWD tool 22. In some embodiment, such transmitter coils operate at a frequency in the range of 200 Kilohertz (KHz) to 2 Megahertz (MHz). A given embodiment may employ one or more transmitter coils configured to operate at a single frequency or multiple frequencies within this range.

In some embodiments, LWD tool 22 enables the transmitter coil by providing a sinusoidal current at a chosen frequency (e.g., within the range mentioned above) to the transmitter coil, which in turn causes the transmitter coil to generate the magnetic field. In many embodiments that employ multiple transmitter coils, the transmitter coils are operated in an interdependent manner (as opposed to being operated independently, where each transmitter coil receives an independently generated sinusoidal current, which may be done in alternative embodiments). Thus, the same sinusoidal current can be provided to more than one transmitter coil in at least some such embodiments.

The receiving sensors described herein can be implemented using any sensor that is capable of detecting the magnitude and/or phase of a magnetic field. In one embodiment, the receiving sensors are coils (e.g., inductors) made of insulated copper wire. Ferromagnetic material can be placed inside each coil (e.g., to provide enhanced effective cross-section area). In alternative embodiments, the receiving sensors are Hall-effect sensors. Other types of appropriate sensors may also be used.

FIG. 2A illustrates an axial view of a LWD tool having a sensor arrangement that includes four transmitter coils and two sets of receiving sensors, each containing four sensors. As shown, LWD tool 22 is implemented as a drill collar that includes a drilling mad channel 102 along the long axis of LWD tool 22 to allow drilling mud (a drilling fluid) to flow to the drill bit.

Four transmitter coils 104 are distributed around the circumference of the LWD tool, at a position that is closer to one end (shown towards the left hand side of FIG. 2C) of LWD tool 22. Due to the orientation of LWD tool 22 in this example, only three of the transmitter coils are visible. These transmitter coils 104 are located within the same plane, which forms a transverse (perpendicular to the long axis of LWD tool 22) cross section of LWD tool 22. Transmitter coils 104 are positioned such that the long axis of each transmitter coil is parallel to the long axis of LWD tool 22.

In this example, these transmitter coils are located within recesses 106 in the body of LWD tool 22. These recesses can be machined into the body of LWD tool 22. The recesses can be filled with non-conductive material. The transmitter coils can be insulated from each other, as well as from the receiver sensors.

Two sets of receiver sensors are distributed around the circumference of the other end (away from the transmitter coils) of LWD tool 22. The receiver sensors are configured to detect the magnetic field induced by the transmitter coils. Since the receiver sensors are detecting a magnetic field, there is no need for a physical electrical connection to couple the transmitter coils and receiver sensors.

In one embodiment, the receiver sensors are located approximately 2 meters from the transmitter coils. Each set of receiver sensors is located in a respective plane, each of which forms a transverse cross section of LWD tool 22. Receiver sensors 108 include four sensors (only three of which can be seen in the view of FIG. 2A) that are oriented such that their axis is parallel to the long axis of LWD tool 22 and the long axis of each of the transmitter coils. Receiver sensors 108 are located within recesses 110 in the body of LWD tool 22. Like the recesses that house the transmitter coils, recesses 110 can be filled with non-conductive material.

The second set of receiver sensors, receiver sensors 112, is also distributed around the circumference of LWD tool 22. Receiver sensors 112 are placed in a different transverse cross sectional plane of LWD tool 22 than receiver sensors 108. Receiver sensors 112 are oriented such that a long axis of each of receiver sensors 112 is perpendicular to the long axis of LWD tool 22, as well as to the long axis of each of receiver sensors 108 and the long axis of each of the transmitter coils 104. Receiver sensors 112 are located within recesses 114 in the body of LWD tool 22. Like the recesses that house the transmitter coils and receiver sensors 108, recesses 114 can be filled with non-conductive material.

Each of receiver sensors 108 and 112 is configured to operate independently. Accordingly, each receiver sensor can independently measure a magnetic field, such that each receiver sensor obtains a different measurement of the same magnetic field. Thus, the receiver sensors 108 and 112 can each be insulated from each other, as well as from transmitter coils 104.

Receiver sensors 108 and 112 are each configured to output an electrical signal indicative of the magnitude and/or phase of a detected magnetic field. In one embodiment, each of receiver sensors 108 and 112 is a receiver coil. The magnetic field induces a voltage in these coils. The magnitude and phase of this induced voltage in the receiver coils, relative to the current (which is used to enable the transmitter coils to generate the magnetic field) in transmitter coils 104, provides a measure of the electrical resistivity of the surrounding formation. The phase difference and/or amplitude ratio of the induced voltages in (or resistivity measured at) a pair of the receiver sensors can be used to determine the resistivity of the portion of the formation that lies between the two receiver sensors in the pair.

The orientation of each receiver sensor's long axis, relative to the orientation of the transmitter coil(s), determines the portion of the magnetic field that the receiver sensor will be sensitive to, as will be explained in the context of FIG. 6. FIG. 6 illustrates an LWD tool 22 that is moving through a formation 404. An anomalous formation 406 is located at a distance from the LWD tool 22. LWD tool 22 induces a primary magnetic field 400 (for simplicity, only a representation of the magnetic component of this field is included) in the surrounding formation. Primary magnetic field 400 propagates in all directions with axial symmetry. The primary magnetic field is affected by surrounding formation 404, and thus the measurement of the primary magnetic field detected by the receiving sensors indicates the resistivity of the surrounding formation 404.

Since primary magnetic field 400 is an alternating field which propagates through the rock media, it is partially reflected by the boundary between media of differing resistivities. Accordingly, the primary magnetic field is reflected by the anomalous formation 406. The resulting reflected field 402 alters primary magnetic field 400, tilting the angle of the primary magnetic field's vector by an amount that is related to the relative strength of the reflection and the distance to the boundary between surrounding formation 404 and anomalous formation 406. The effect of the reflected magnetic field on the primary magnetic field is more pronounced closer to the boundary between the two formations, such that a receiver sensor closer to the boundary will respond to a stronger component of the reflected magnetic field than a receiver sensor further from the boundary.

The receiver sensors that are oriented in the same manner as the transmitter coils will be primarily sensitive to the primary magnetic field. These receiver sensors will also be sensitive to the reflected magnetic field. In particular, these receiver sensors (e.g., receiver sensors 108 of FIG. 2A) will detect the reflected magnetic field via its effect on the primary magnetic field.

The receiver sensors (e.g., receiver sensors 112 of FIG. 2A) that are oriented with their long axes perpendicular to the orientation of the transmitter coils are orthogonal to the primary magnetic field. Accordingly, these sensors will not respond to the primary magnetic field. However, these sensors will be sensitive to the reflected magnetic field. Accordingly, embodiments that include these sensors can provide an increased sensitivity to the reflected magnetic field, relative to the sensors that are configured to respond primarily to the primary magnetic field.

As LWD tool 22 moves through the borehole, LWD tool 22 can detect changes in the resistivity of the surrounding formation. Such changes in resistivity indicate that the surrounding formation is changing, since different types of geological formations have different resistivities. Thus, the presence of an anomalous formation within the surrounding formation will cause the detected resistivity to change as the drill bit nears the anomalous formation. The measured resistivity can be used to determine the type of geological formation surrounding the drill bit, as well as the type and/or location of anomalies within the surrounding geological formation, as will be discussed in more detail below.

FIG. 2B shows an alternative sensor arrangement. In this example, LWD tool 22 includes four sensors 108 that are oriented such that the long axis of each sensor 108 is parallel to the long axis of LWD tool 22 and each of the transmitter coils 106. Receiver sensors 108 are sensitive to both the primary and reflected (if any) magnetic fields.

FIG. 2C shows yet another alternative sensor arrangement. In this example, LWD tool 22 includes four sensors 108 that are oriented such that the long axis of each sensor 108 is perpendicular to the long axis of LWD tool 22 and each of the transmitter coils 106. In this example, the receiver sensors 108 are sensitive to the reflected magnetic field (if any) but not to the primary magnetic field.

The sensor arrangements of FIGS. 2A-2C allow each set of sensors 108 and/or 112 on LWD tool 22 to simultaneously obtain measurements of the relative magnitude and/or phase of a magnetic field at four distinct points that are evenly distributed around the circumference of the drill collar. Accordingly, the magnitude and/or phase of the magnetic field surrounding LWD tool 22 can be sensed in each of four directions simultaneously. This allows an operator (i.e., a person controlling the steering of a drill bit to which LWD tool 22 is coupled) to use LWD tool 22 to determine the location of anomalous formations, if any, that the drill bit may be approaching in any of those four directions, without rotating LWD tool 22. Reducing the need to rotate the LWD tool 22 can simplify and/or speed up the operation of LWD tool 22.

As noted above, many other sensor configurations are possible, in addition to those shown in FIGS. 2A-2C. For example, one alternative embodiment may employ only two receiver sensors (or two sets, each having two receiver sensors, where the axis of the sensors in one set is perpendicular to the axis of the sensors in the other set). When in use, an operator may rotate the LWD tool 180 degrees in between two sets of measurements at a given location, in order to obtain the same measurements that could be obtained simultaneously if four receiving sensors were used.

Other numbers of receiver sensors can also be included within each set, so long as each set includes at least two sensors. For example, some embodiments may include three sensors, while other embodiments include eight sensors.

As another example, instead of having each sensor within a given set of sensors (where each set contains sensors having substantially the same axis orientation, relative to the axis of the drill collar) in substantially the same cross-sectional plane of the drill collar, some sensors in a given set are located in a different cross-sectional plane than other sensors in the same set. Similarly, in some alternative embodiments, sensors from different sets may be arranged in substantially the same cross-sectional plane of the drill collar.

Another variation in sensor configuration can switch the location of the transmitter coils and one or both sets of receiver coils. Thus, some embodiments may place transmitter coils closer to the drill bit and receiver sensors towards the other end (furthest from the drill bit), while other embodiments place one or both sets of receiver sensors closer to the drill bit while placing the transmitter coils further from the drill bit. Similarly, in one embodiment, the transmitter coils may be located towards the middle of the drill collar, and one or more sets of receiver sensors may be located towards each end of the drill collar, such that the transmitter coils are between two sets of receiver sensors. Still another variation in sensor arrangement can orient the transmitter coils such that the long axis of the transmitter coils is perpendicular to the long axis of the LWD tool.

FIG. 3 illustrates a cross-sectional view of LWD tool 22. This view shows the drilling mud channel 102 passing through the middle of LWD tool 22. Four recesses, 110(a), 110(b), 110(c), and 110(d), extend into the body of LWD tool 22. These recesses are all located in the same cross-sectional plane. These recesses are spaced 90 degrees apart from each other around the circumference of LWD tool 22.

A receiver sensor (e.g., one of receiver sensors 108 of FIGS. 2A and 2C) has been inserted into each recess. In particular, receiver sensor 108(a) is located within recess 110(a), receiver sensor 108(b) is located within recess 110(b), receiver sensor 108(c) is located within recess 110(c), and receiver sensor 108(d) is located within recess 110(d). The receiver sensors are oriented such that the axis of each sensor is parallel to the axis of LWD tool 22.

A similar spacing and arrangement (now shown in FIG. 3) of sensors can be used to hold sensors (e.g., such as sensors 112 of FIGS. 2A and 2C) that are oriented such that the axis of each sensor is perpendicular to the axis of LWD tool 22. Thus, appropriate recesses to hold sensors oriented in that manner can be located in the same cross-sectional plane, spaced 90 degrees apart from each other.

Different embodiments can vary from the spacing described herein. For example, due to imprecision in measurement and/or machining tools, it is often impractical to attempt to obtain perfectly even spacing. Accordingly, most embodiments allow for some margin of error (e.g., 0.5%, 1%, or 5%, depending on the measurement and/or machining tools available and/or the desired accuracy of the resulting LWD tool) in the spacing and/or orientation of the transmitting coils and/or receiver sensors.

In some embodiments, even greater variations in spacing are used. Instead of having each sensor be arranged at a position that is spaced approximately 90 degrees from two other sensors, alternative embodiments may locate sensors around the circumference of the drill collar using a more varied spacing. For example, in one alternative embodiment, each sensor may be located closer than 90 degrees (e.g., 80 degrees) to one neighboring sensor and more than 90 degrees (e.g., 100 degrees) from the other neighboring sensor.

The position of each sensor can be mechanically keyed to a separate directional sensor or orientation sensor (e.g., a gyroscope). This allows the signal detected by each sensor to be analyzed as a component of a vector electromagnetic field, which has a direction referenced to the gravity vector and/or the direction of the magnetic north pole, or to an inertial reference vector.

FIG. 4 is a flowchart of a method of using a LWD tool that has a sensor arrangement like one of those described above, in which more than one receiver sensor independently obtains a measurement of a magnetic field. This method can be performed by a LWD tool 22 as described in the figures above, in combination with components such as those shown in FIG. 5 (if such components are not integrated into LWD tool 22).

The method begins at 200, when a magnetic field is induced in a formation. As noted above, inducing a magnetic field can involve applying a sinusoidal current to one or more transmitter coils at a prespecified frequency. This operation can be performed by a current generator (e.g., an oscillator operating under the control of a control module, which can in turn be responsive to an operator's commands to take a measurement), operating in conjunction with one or more transmitter coils that are coupled to receive the sinusoidal current produced by the current generator.

At 205, electrical signals (e.g., a voltage induced in two or more receiver sensors) are received. The electrical signals indicate the magnitude and/or phase of a reflected magnetic field at multiple locations around the drill collar. In particular, each electrical signal indicates the magnitude and/or phase of a reflected magnetic field, as measured at a respective one of several locations along the circumference of the drill collar. These electrical signals are generated by two or more receiver sensors and received by components such as those shown in FIG. 5. As noted above, the magnitude and/or phase of the reflected magnetic field can be detected by measuring the reflected magnetic field directly (e.g., through the use of sensors that are oriented with their axes perpendicular to the orientation of the transmitter coil(s)' axes) or indirectly (e.g., through the use of sensors that are oriented with their axes parallel to the orientation of the transmitter coil(s)' axes, where such sensors measure the reflected magnetic field based upon its effect on the primary magnetic field).

At 210, a vector representation of the reflected magnetic field is calculated, based upon the electrical signals received at 205. This vector representation can be calculated by a control module (e.g., control module 302 of FIG. 5 below), which can be included within and/or coupled to the LWD tool. This vector representation can then be displayed to an operator, logged, and/or used to generate a more sophisticated display (e.g., such as a polar display for geosteering applications). More details regarding the calculation of this vector representation are provided below. The vector representation indicates the location of (e.g., in terms of an azimuthal angle), distance to, and resistivity of the anomaly. The latter two components of the vector representative can be relative, such that the vector can represent multiple possibilities, including a less resistive anomaly closer to the LWD tool and a more resistive anomaly further from the LWD tool. Historical data about a formation can be used to select a more specific interpretation of the vector.

FIG. 5 is a block diagram of circuitry 300 that can be included within and/or coupled to LWD tool 22. Circuitry 300 includes a control module 302, a power amplifier 304, one or more low-noise amplifiers 306 (only one such low-noise amplifier is illustrated in FIG. 5; however, one such amplifier can be included for each receiver sensor), a switching module 308, a switching module 312, an oscillator 310, signal mixer, detector, and filter 314, analog-to-digital converter 316, and transmission module 318. Circuitry 300 can also include one or more memory devices such as memory 320.

Oscillator 310, switching module 310, and power amplifier 304 operate to enable one or more transmitter coils in order to induce a magnetic field in a formation surrounding the LWD tool. In particular, oscillator 310 (or any other appropriate current generator) generates a sinusoidal current (e.g., in the range of 100-400 KHz), which is then provided to power amplifier 304 via switching module 312. In this embodiment, control module 302 controls switching module 312 in order to select when the sinusoidal current is provided to power amplifier 304 (e.g., the current can only be provided when an operator is requesting that the magnetic field be induced in the surrounding formation for testing purposes). Power amplifier 304 amplifies the sinusoidal current, which is then provided to one or more transmitter coils (not shown) in the LWD tool.

The frequency of oscillator 310 can be selected based upon the desired responsiveness of the LWD tool. For example, in many situations, it is desirable to maximize the distance from the borehole at which the LWD tool is able to sense an anomalous formation, since this will give a driller early warning that he may soon encounter changed formation conditions. The radial depth of investigation is strongly influenced by the phenomenon of “skin depth,” which is a characteristic distance that an alternating magnetic field can penetrate into a conductive medium. The skin depth is defined as the distance at which an electromagnetic field has experienced a phase-shift of 45 degrees and an attenuation of 1/e (−8.68 dB). Skin depth may be calculated as follows: Skin Depth, δ(meters)=(2/ω·μ·σ)^(1/2)  (Eq. 1) where ω is the angular frequency (radians/second), μ is the magnetic permeability of the medium (Henries/meter), and σ is the conductivity of the medium (Siemens/meter).

Since the field must penetrate into the formation and be reflected back to be detected by the receiving sensors, the field effectively makes two trips, so the attenuation and phase-shift of the received signal will be doubled when the investigation distance equals the skin-depth. A practical limit for detection distance is when the attenuation reduces signal levels below the noise or accuracy limit of the measurement circuits attached to the receiving coils. To maximize the skin-depth in a given medium, one can choose to minimize the frequency since this is the only independent parameter. However, the magnitude of the signal induced in a receiving sensor is proportional to frequency, so a best compromise must be found in the tool design. Such a compromise can be found by using a frequency in the range of 100-400 KHz for embodiments in which the transmitter coils are spaced a few meters or so from the receiver sensors. At a frequency of 200 KHz, the skin depth ranges from 0.4 to 40 meters for a typical rock resistivity range of 0.1 to 1000 ohm-meters. Thus, the potential depth of investigation could be at least several meters in the higher resistivity range typical of most hydrocarbon-bearing reservoir rocks.

A voltage, which is dependent upon the magnetic field in the surrounding formation, can be induced in each receiver sensor. Each of the receiver sensors (not shown) is coupled to a respective low noise amplifier 306, which amplifies the signal induced in the respective receiver sensor and provides that amplified signal to switching module 308. Control module 302 controls switching module 308 (e.g., in order to select which receiver sensor's output is input to signal mixer, detector, and filter 314 at any given time).

The output of switching module 308 is provided to signal mixer, detector, and filter 314. Signal mixer, detector, and filter 314 also receive a reference signal (f_(ref)) (e.g., indicating the signal provided to the transmitter coils) from oscillator 310. Signal mixer, detector, and filter 314 can, among other things, remove unwanted noise from the amplified signal. For example, signal mixer, detector, and filter 314 can remove unwanted noise from image frequencies.

Signal mixer, detector, and filter 314 provides its output to analog-to-digital converter 316, which in turn outputs a digital representation of the signal received from signal mixer, detector, and filter 314 to control module 302. Control module 302 then processes this digital signal (in conjunction with other digital signals representing the signals detected at the other sensors included in the LWD tool) in order to produce a vector representation of the reflected magnetic field. In particular, control module 302 can obtain the magnitude and/or phase of the signal detected at each sensor, as well as positional information identifying the location of each sensor (e.g., relative to the gravity vector, magnetic North, or the axis of the LWD tool), in order to obtain a vector representation of the magnetic field. This vector representation can then be used to determine the resistivity and location (e.g., in terms of direction and distance) of an anomalous formation. Control module 302 can also process the received digital signals in order to identify the resistivity of the surrounding formation in the absence of any anomaly.

In one embodiment, the magnitude and phase of the induced voltage in a pair of receiver sensors (a pair includes two sensors in the same set, which includes sensors having the same orientation, that are spaced approximately 180 degrees apart from each other on the circumference of the LWD tool), relative to the current in the transmitter coil, is an accurate measure of the electrical resistivity of the rock in a selected location. Accordingly, the phase difference or the amplitude ratio between signals from a pair of closely-spaced receiver sensors (e.g., any two of the receiver sensors in a given set of similarly oriented receiver sensors) provides an accurate indication of rock resistivity in a narrow slice of rock between the sensors. Thus, control module 302 can calculate a value that is representative of the formation resistivity, based upon the signals received from the receiver sensors.

In particular, for a pair of sensors having axes parallel to the axis of the transmitter coil(s), the vector representation of the magnetic field can be calculated by calculating the ratio of the signals obtained by that pair of sensors. This representation also indicates the resistivity of the formation generally located in the axial region between the sensors. The signals received by these sensors, which are responsive to both the primary and reflected (if any) magnetic fields, can be used to calculate the resistivity of both a homogeneous geological formation and an anomalous geological formation (if any).

If instead the pair of sensors have axes that are perpendicular to the axis of the transmitter coil(s), so that the sensors are only sensitive to a reflected magnetic field, if any, the signals from these sensors can be used to determine the resistivity of an anomalous formation (e.g., by calculating the ratios of signals received by pairs of sensors).

If two sets of sensors, which are oriented perpendicular to each other, are included, the signals received from one set of sensors (e.g., those sensors sensitive to the primary field) can be used to calculate the resistivity of a homogeneous surrounding geological, while the signals received from the other set of sensors can be used to calculate the resistivity of the anomalous geological formation.

In general, when a receiver sensor is spaced apart from a transmitter coil, the voltage V induced in a receiving sensor due to a sinusoidal current I flowing in the transmitter coil is: V(volts)=−jωM·(1−jkL)·e ^(jkL) ·I(amperes)  (Eq. 2) where j is the imaginary operator (−1)^(1/2), M is the mutual inductance between the transmitter and receiver (Henries), L is the spacing between the transmitter and receiver (meters), k is the complex propagation coefficient of the medium, k=[jωμ(σ+jω∈)]^(1/2), and ∈ is the dielectric permittivity of the medium (Farads/meter).

Assuming that the dielectric permittivity of the medium has negligible effect on propagation, then the propagation coefficient reduces to the form: k=(jωμσ)^(1/2)  (Eq. 3) k=(1+j)/δ  (Eq. 4)

Substituting this into Eq. 2 and re-arranging terms results in the simplified form: V/I=−jωM·[1−(j−1)L/δ]·e ^((j−1)L/δ)  (Eq. 5)

In Eq. 5, V/I represents a transfer-function between coils, jωM represents the low-frequency mutual coupling, [1−(j−1)L/δ] represents the spreading effect (since this example is focused on operating in the near-field region of the transmitter and receivers), and e^((j−1)L/δ) represents the plane-wave propagation in the medium. L/δ is a dimensionless term, since it is a ratio between two physical distances.

Control module 302 can be configured to use Eq. 5 to predict the phase-shift and attenuation of the transfer-function in a medium of varying conductivity. Circuitry 300 can be configured to also use Eq. 5 to predict the reflected field from a rock-bed boundary (e.g., the boundary between a surrounding formation and an anomalous formation) in proximity to the LWD tool. If the boundary is parallel to the tool axis and at a distance of D meters, then the total distance from transmitter coil to the boundary and back to the receiver sensor is a matter of geometry: Total Distance D′=2[D ²+(L/2)²]^(1/2)  (Eq. 6)

Substituting D′ for L in Eq. 5 allows calculation of the reflected component of the signal, assuming that the reflection coefficient of the boundary is nearly −1, which is true if there is a large ratio (>10) between the conductivities of the two media on either side of the boundary. If we are only concerned with the phase of a received signal relative to the phase of the induced primary magnetic field, then the phase velocity of the field is constant in the medium regardless of distance, so the phase-shift may be simply represented as: Phase-shift Θ=πD′/4δ  (Eq. 7)

If the receiver sensor is oriented in a manner (e.g., such as shown in FIG. 2C) that is only responsive to a reflected signal from a boundary (i.e., and not to a signal in which the reflected signal is added to a direct signal, as shown in FIG. 2B) then it is easy to invert this equation to determine distance from the phase measured by that receiver sensor: Distance to Boundary D=[(2Θδ/π)²−(L/2)²]^(1/2)  (Eq. 8)

The resistivity of the rock surrounding the tool can be determined from the simultaneous measurement with receiver sensors that are oriented in the same manner as the transmitter coils (e.g., as shown in FIG. 2B). For example, if all of those receiver sensors detect approximately the same resistivity value, then that value indicates the resistivity of the surrounding formation. If the sensors begin to detect different values, the most recent consistent (among those sensors) value can be used as the resistivity of the surrounding formation, and the difference among sensor outputs can be attributed to an anomalous formation that the LWD tool is approaching. A value indicating the resistivity of the surrounding formation (or a log of such values, and the time (and/or location) at which each value was obtained) can be stored in memory 320.

Control module 302 can use that resistivity of the surrounding formation to compute the skin-depth to be used in Eq. 8, and hence find the value of D. In some embodiments, an advanced 3-dimensional Finite-Element computer-modeling program such as FEMLAB™, available from Comsol, Inc. of Burlington, Mass., may be employed to simulate a wide range of formation conditions and to create a database, allowing control module 302 to perform inversion of measured phase-shift to actual boundary distance using the measured phase-shift and the database of simulated formation conditions. Such simulated results can be stored in memory 320 (e.g., in the form of a lookup table), allowing control module 302 to lookup a measured phase shift and obtain an appropriate distance to a boundary.

Thus, control module 302 can detect whether the LWD tool (and thus the drill bit to which the LWD tool is attached) is approaching or moving away from an anomaly. For example, as described above, control module 302 can compare the most recently calculated set of apparent resistivities detected by each sensor to historical resistivity values (e.g., generated by a computer-modeling program) stored in memory 320. If any of the resistivities have increased relative to the historical resistivities, control module 302 can determine that the LWD tool is approaching a more resistive anomaly. Similarly, if any of the detected resistivities have decreased relative to the historical resistivities, control module 302 can determine that the LWD tool is approaching a less resistive anomaly.

The output from a directional sensor included in and/or coupled to the LWD tool can then be used to determine the relationship between that known point and a standard directional vector, such as magnetic north or the gravity vector. The output from the directional sensor can thus be provided to control module 302, which can use this information to calculate a relative azimuthal angle describing the location of the anomaly relative to the standard directional vector.

More exact techniques can be used to calculate the azimuthal angle and distance to a resistivity anomaly using the configuration of the sensors. An example is described below with respect to FIG. 7.

FIG. 7 illustrates a vector representation of a reflected magnetic field (produced by, for example, a resistivity anomaly such as a rock-bed boundary between two rocks of contrasting resistivity) relative to a LWD tool 22. As shown, the LWD tool 22 includes a pair of sensors, labeled X and Z. Sensor X has a long axis that is perpendicular to the axis 700 of LWD tool 22 and a transmitter coil. Sensor Z has a long axis that is parallel to axis 700 of LWD tool 22 and the transmitter coil.

The magnetic field vector (shown as a bold arrow pointing towards LWD tool 22) approaches LWD tool at a relative angle θ that is defined with respect to axis 700 of LWD tool 22. The magnetic field vector is normal to the surfaces of constant magnetic field strength, and indicates the general direction of a resistivity anomaly such as a bed-boundary that attenuates and reflects a magnetic field generated by a transmitter within LWD tool 22.

A voltage is induced in each of sensors X and Z by the magnetic field. The voltage induced in sensor X is V_(x) and the voltage induced in sensor Z is V_(z). These voltages can be used to calculate the relative angle θ as the arctangent of the ratio of V_(x) to V_(z), such that θ=arctangent (V_(x)/V_(z)). This angle represents the relative angle in the plane defined by the X and Z axis. This technique allows the relative angle to be detected, while also providing a robust LWD tool assembly since very little metal needs to be removed from the circumference of the collar to allow for insertion of the sensors.

If there is a similarly oriented sensor Y (not shown in FIG. 7), located 90° away from sensor X along the circumference of LWD tool 22 and having its long axis perpendicular to the Z sensor axis (and thus being oriented similar to sensor X), a second angle Φ can be calculated as the arctangent of the ratio of the voltage V_(y) induced in sensor Y to V_(z), such that Φ=arctangent (V_(y)/V_(z)). This angle represents the relative angle of the vector in a plane defined by the Y and Z axis.

The two angles together specify the direction of the magnetic field vector relative to axis 700 of LWD tool 22. In some embodiments, additional calculations can be applied to the calculated angles and/or during the calculation of these angles. For example, the voltages induced in the sensors can be scaled to account for differences in the gain of each of the sensors, and also to correct for the influence of the conductive drill collar of LWD tool 22, which can perturb the magnetic field detected by all or some of the sensors. The corrections can be derived from computer modeling of the performance of LWD tool 22 in various situations.

The two angles can be combined with the distance estimate described above to provide a complete vector representation, in terms of distance, magnitude, and angle, of the magnetic field. The magnitude V_(m) of the magnetic field is equal to the square root of the sum of V_(x) squared, V_(y) squared, and V_(z) squared. Again, certain scaling factors can be applied to the component voltages prior to calculating V_(m), depending upon the particular configuration of the sensor coils being used.

Another more general solution of the measured voltages V_(x), V_(y) and V_(z) in a polar coordinate system is more useful in some situations. We define an azimuthal angle (here, an angle of rotation around the Z axis relative to a fixed reference-point) as AZI, and an angle relative to the Z axis in any azimuthal direction (i.e., a relative bearing) as RB. Then AZI=arctangent (V_(y)/V_(x)) and RB=arccosine (V_(z)/V_(m)). With this solution, it is not necessary to rotate the drill-collar to obtain a complete solution of the direction of the magnetic vector.

Using the AZI angle and the Distance D as described previously, the data presentation method described in copending U.S. patent application Ser. No. 11/756,504, titled “Azimuthal Measurement While Drilling (MWD) Tool,” filed May 31, 2007, and naming Paul L. Sinclair and Thomas A. Springer as inventors, can be used to generate a polar display. This application is hereby incorporated by reference as if completely and fully set forth herein.

Returning to FIG. 5, transmission module 318 is configured to communicate information received and/or calculated by control module 302 to another sub or to a surface system. For example, transmission module 318 can receive information generated by control module 302 (e.g., by receiving the information directly from control module 302 or by retrieving such information from a storage device such as memory 320) that indicates the resistivity of the surrounding formation and transmit this information to a surface system. Alternatively, transmission module 318 can transmit information indicative of the magnitude and/or phase of the reflected electromagnetic field detected at each sensor to a surface system, allowing the surface system to calculate the resistivity of the surrounding formation from the data collected by LWD tool 22.

Transmission module 318 can also receive information sent to control module 302 by a surface system and/or another sub. For example, transmission module 318 can receive information indicating that a surface user would like LWD tool 22 to begin measuring the resistivity of the surrounding formation. Transmission module 318 can provide this information to control module 302 and/or store this information for subsequent access by control module 302. Transmission module 318 can be configured to interface to and/or communicate via a wire line cable (e.g., wire line cable 24 of FIG. 1), a telemetry system, or any other desired communication system and/or communication media.

In some embodiments, at least some of the components shown in FIG. 5 are included within the drill collar implementing LWD 22. For example, at least some of these components can be placed within one or more cavities within the drill collar. Alternatively, all or some of these components can be located within a cartridge that is configured to be coupled to or located within the drill collar (e.g., such a cartridge can be suspended within the drilling mud channel). In one embodiment, such a cartridge includes components such as electronic circuits, communication circuits, directional sensors (e.g., configured to detect a standard directional vector, such as the gravity vector or magnetic North), and the like. In some embodiments, this inner cartridge is retrievable, such that the inner cartridge can be installed and/or withdrawn from LWD tool 22 while LWD tool 22 is below the surface. For example, the inner cartridge can be installed or withdrawn through the drillstring using a slick line cable or wireline attached to the upper end of the LWD tool. The drillstring can include a muleshoe (not shown) to accept and orient the inner cartridge in such an embodiment.

While certain components are shown as part of the LWD tool in FIG. 5, it is noted that in alternative embodiments, such components can be implemented within other subs within the drillstring and/or other components within the drilling system. For example, the transmission module 318 and/or directional sensors (not shown) can each be implemented within another sub. Similarly, all or part of the functionality of control module 302 can be implemented within another sub or within a surface computing device (e.g., a laptop computer). For example, in one embodiment, a portion of control module 302 (e.g., the portion that calculates the vector representation of the electromagnetic field detected at each sensor) is implemented in software executing on a computer system located at the surface. Another portion of the control module 302 (e.g., the portion that controls the operation of oscillator 310, switching module 312, analog-to-digital converter 316, and transmission module 318) can be implemented in hardware, firmware, and/or software residing in LWD 22.

Alternatively, in some embodiments, an LWD tool may store and transmit only the digitized raw measurements from the sensors, for more detailed computations using a computer at a surface location, and thus the majority, or even all, of control module 302 may be implemented at such a surface computer. In still other embodiments, the LWD tool will be designed to process and transmit some of the data as described, but due to the limited data-rate of transmission available, the complete set of measured data will be stored (e.g., in Flash Memory) within the tool for later download to a surface computer when the borehole assembly is returned to the surface (e.g., during a bit run).

While specific equations have been described in the above description of FIG. 5, it is noted that other embodiments can implement different equations. For example, a LWD tool having six sensors will use different equations than those presented above, which are designed for use with a LWD tool having four sensors.

It is noted that all or some of the control module 302 shown in FIG. 5 can be implemented in software executing on a computing device (e.g., a personal computer, server, personal digital assistant, cell phone, laptop, workstation, or the like). In particular, such a computing device includes one or more processors (e.g., microprocessors, PLDs (Programmable Logic Devices), or ASICs (Application Specific Integrated Circuits)) configured to execute program instructions stored in a memory. Such a memory can include various types of RAM (Random Access Memory), ROM (Read Only Memory), Flash memory, and the like. The computing device can also include one or more interfaces (e.g., such as network interfaces, one or more interfaces to storage devices, and/or one or more interfaces to an input/output (I/O) device such as a keyboard, digital tablet, mouse, monitor, or the like), which can each be coupled (e.g., by a bus or other interconnect) to the processor(s) and memory.

It is noted that the program instructions and data (e.g., such as a history log and/or lookup table) consumed by and/or implementing all or part of control module 302 can be stored on various computer readable media such as memory 320. In some embodiments, such program instructions can be stored on a computer readable storage medium such as a CD (Compact Disc), DVD (Digital Versatile Disc), hard disk, optical disk, tape device, floppy disk, and the like. In order to be executed by a processor, the instructions and data are loaded into memory from the other computer readable storage medium. The instructions and/or data can also be transferred to a computing device for storage in memory via a network such as the Internet or upon a carrier medium.

FIG. 8A illustrates how a sensor can be placed in the wall of a LWD tool 22. The view in FIG. 8A looks at the surface of the drill collar, and the long axis of the sensor is represented by the dashed arrow. FIG. 8B shows a cross-sectional view of the drill collar, when viewed in direction A-A marked on FIG. 8A. As shown in these figures, a meandering channel 820 has been formed in the drill collar. In addition, a cross-drilled hole 840 provides a location for a receiver sensor, as shown in cross-section in FIG. 8B, and the combination of the meandering channel 820 and the hole 840 operatively breaks the path of continuous metal around sensor 830. A wiring channel 810 connects this channel to an electrical connector 800, which can provide a voltage induced in a receiver sensor 830 disposed in a transverse hole 840 in the drill collar to other circuitry (e.g., circuitry 300 of FIG. 5) for further processing.

Meandering channel 820 and wiring channel 810 can be filled with non-conducting material, such as a composite epoxy/ceramic material or an elastomer which surrounds and protects a receiver sensor 830 in hole 840, and wires (in wiring channel 810) coupling the receiver sensor to electrical connector 800.

Using a meandering channel (as opposed to a non-meandering channel having the maximum width and length of the meandering channel) retains more of the metal of the drill collar. By retaining more of the metal in the drill collar, the drill collar's strength can be relatively less affected by the inclusion of the receiver sensor. Additional, the retained metal can provide increased protection to receiver sensor 830 from abrasions and impacts that occur during drilling. At the same time, the meandering channel ensures that there is no electrically conductive path around receiver sensor 830. If present, such an electrically conductive path could short-circuit receiver sensor 830.

FIGS. 8A and 8B show how a sensor having its long axis perpendicular to the axis of the drill collar can be protected by a meandering channel machined into the surface of the drill collar. A similar configuration, rotated 90° with respect to the orientation of the sensor axis shown in FIG. 8A, can be used to protect a sensor having its long axis parallel to the axis of the drill collar.

Although the present invention has been described in connection with several embodiments, the invention is not intended to be limited to the specific forms set forth herein. On the contrary, the present invention is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A method comprising: transmitting a primary magnetic field from one or more coils placed on a drill collar; receiving a plurality of electrical signals, wherein each of the electrical signals is received from a respective one of a plurality of sensors, wherein the sensors are distributed around a circumference of a drill collar, wherein an axis of at least one of the sensors is perpendicular to an axis of the drill collar, wherein each of the electrical signals indicates a respective magnitude of a measurement of a reflected magnetic field, wherein the reflected magnetic field is reflected from an anomalous geological formation; and calculating a vector measurement of the reflected magnetic field, based upon the electrical signals.
 2. The method of claim 1, wherein each of the electrical signals also indicates a phase of the reflected magnetic field, relative to the primary magnetic field.
 3. The method of claim 1, wherein an axis of a second one of the plurality of sensors is parallel to an axis of a transmitter coil configured to induce the primary magnetic field.
 4. The method of claim 3, wherein the calculating comprises: calculating a plurality of ratios, based upon the electrical signals; and calculating the vector measurement, dependent upon the ratios.
 5. The method of claim 3, further comprising: calculating an electrical characteristic of a homogeneous geological formation and the electrical characteristic of the anomalous geological formation, based upon a combination of the electrical signals.
 6. The method of claim 1, wherein an axis of each of the plurality of sensors is perpendicular to an axis of a transmitter coil configured to induce a primary magnetic field.
 7. The method of claim 1, further comprising: receiving a plurality of second electrical signals, wherein each of the second electrical signals is received from a respective one of a plurality of second sensors, and wherein the second sensors are distributed around the circumference of the drill collar at a distance from the sensors, and wherein an axis of each of the second sensors is perpendicular to an axis of each of the sensors.
 8. The method of claim 7, further comprising: calculating an electrical characteristic of a homogeneous geological formation based upon the electrical signals; and calculating the electrical characteristic of the anomalous geological formation, based upon the second electrical signals.
 9. The method of claim 1, further comprising: accessing information indicative of a resistivity of the anomalous geological formation; and calculating a distance from the drill collar to the anomalous geological formation, based upon the vector measurement and the information.
 10. The method of claim 9, wherein the information is calculated based upon a plurality of second electrical signals, and wherein each of the second signals is received from a respective one of a plurality of second sensors.
 11. A system comprising: a drill collar; one or more coils configured to generate a primary magnetic field, wherein the one or more coils are distributed around a circumference of the drill collar; a plurality of sensors distributed around a circumference of the drill collar, wherein an axis of at least one of the sensors is perpendicular to an axis of the drill collar, wherein the plurality of sensors is configured to output a plurality of electrical signals, and wherein each of the electrical signals indicates a respective magnitude of a measurement of a reflected magnetic field, wherein the reflected magnetic field is reflected from an anomalous geological formation; and a control module coupled to receive information identifying the plurality of electrical signals and to calculate a vector measurement of the reflected magnetic field, based upon the information.
 12. The system of claim 11, wherein the control module is located within the drilling subassembly.
 13. The system of claim 11, wherein each of the electrical signals also indicates a phase of the reflected magnetic field relative to the primary magnetic field.
 14. The system of claim 11, wherein each of the electrical signals also indicates a magnitude of the reflected magnetic field.
 15. The system of claim 11, wherein an axis of a second one of the plurality of sensors is parallel to an axis of a transmitter coil configured to induce the primary magnetic field.
 16. The system of claim 15, wherein the control module is configured to: calculate a plurality of ratios, based upon the electrical signals; and calculate the vector measurement, dependent upon the ratios.
 17. The system of claim 15, wherein the control module is configured to calculate an electrical characteristic of a homogeneous geological formation and the electrical characteristic of the anomalous geological formation, based upon a combination of the electrical signals.
 18. The system of claim 11, wherein an axis of each of the plurality of sensors is perpendicular to an axis of a transmitter coil configured to induce a primary magnetic field.
 19. The system of claim 11, further comprising: a plurality of second sensors, wherein the second sensors are distributed around the circumference of the drill collar at a distance from the sensors, wherein an axis of each of the second sensors is perpendicular to an axis of each of the sensors, wherein the plurality of second sensors are configured to output a plurality of second electrical signals, and wherein the control module is configured to: receive information identifying the plurality of second electrical signals, calculate an electrical characteristic of a homogeneous geological formation based upon the electrical signals, and calculate the electrical characteristic of the anomalous geological formation, based upon the second electrical signals.
 20. The system of claim 19, further comprising: a plurality of second sensors distributed around the circumference of the drill collar and configured to output a plurality of second electrical signals, wherein the control module is configured to calculate the information indicative of the resistivity, based upon information identifying the plurality of second electrical signals.
 21. The system of claim 19, wherein a metal surface of the drill collar comprises a meandering channel filled with non-conductive material, and wherein the meandering channel overlays the at least one of the sensors.
 22. The system of claim 11, wherein the control module is configured to: access information indicative of a resistivity of the anomalous geological formation; and calculate a distance from the drill collar to the anomalous geological formation, based upon the vector measurement and the information.
 23. A system comprising: means for generating a plurality of electrical signals, wherein the means for generating are distributed around a circumference of a drill collar, wherein an axis of at least one of the means for generating is perpendicular to an axis of the drill collar, and wherein each of the electrical signals indicates a respective magnitude of a measurement of a reflected magnetic field, wherein the reflected magnetic field is reflected from an anomalous geological formation; and means for calculating a vector measurement of the reflected magnetic field, based upon the electrical signals.
 24. The system of claim 23, wherein each of the electrical signals also indicates a phase of the reflected magnetic field relative to a primary magnetic field.
 25. The system of claim 24, wherein the means for calculating further calculate an electrical characteristic of a homogeneous geological formation and the electrical characteristic of the anomalous geological formation, based upon a combination of the electrical signals.
 26. The system of claim 23, further comprising: means for generating a second plurality of electrical signals, wherein the means for generating the second plurality of electrical signals are distributed around the circumference of the drill collar at a distance from the means for generating the plurality of electrical signals, and wherein the means for calculating further calculates an electrical characteristic of a homogeneous geological formation, based upon the electrical signals, and the electrical characteristic of the anomalous geological formation, based upon the second electrical signals.
 27. The system of claim 23, wherein the means for calculating comprises: means for accessing information indicative of a resistivity of the anomalous geological formation; and means for calculating a distance from the drill collar to the anomalous geological formation, based upon the vector measurement and the information. 